This invention relates generally to cardiac rhythm management devices and methods and particularly, but not by way of limitation, to a rate adaptive cardiac rhythm management device using transthoracic impedance information, such as a minute ventilation signal to control the rate at which pacing therapy is delivered to a patient""s heart.
Pacemakers and other cardiac rhythm management devices deliver cardiac therapy to a patient""s heart to assist in obtaining a rhythm of heart contractions that maintains sufficient blood flow through the patient""s circulatory system under a variety of conditions. In particular, rate-adaptive pacemakers deliver electrical pacing pulses to stimulate contractions of the heart. The rate at which the pulses are delivered is adjusted to accommodate a metabolic need of the patient During exercise, higher pacing rates are delivered, while lower pacing rates are delivered when the patient is at rest
Different parameters are used as an indication of the patient""s metabolic need for pacing therapy, including: blood pH blood temperature, elecocardiogram (ECG) artifacts such as QT interval, blood oxygen saturation, breathing rate, minute ventilation, etc. Pacemakers include specific control algorithms for tracking the parameter indicating metabolic need, and providing a control signal for adjusting the pacing rate accordingly. A variety of difficulties exist that complicate sensing of the parameter indicating metabolic need and controlling the pacing rate.
For example, detecting blood pH encounters sensor stability problems. pH sensors may drift with age and time. Blood oxygenation saturation is measured using light emitters that complicate the lead system used to couple the pacemaker""s pulse generator to the heart. Blood temperature is a poor indicator of metabolic need because of the long time lag between the onset of exercise and any detectable increase in blood temperature. ECG artifacts, such as QT interval, are difficult to detect in the presence of other myopotentials and motion artifacts. Breathing rate, also referred to as respiratory rate, is not particularly well correlated with the need for increased blood circulation. For example, it is possible for respiratory rate to increase while the patient is sleeping or talking.
Minute ventilation (also referred to as xe2x80x9cminute volumexe2x80x9d or xe2x80x9cMVxe2x80x9d) is a respiratory-related parameter that is a measure of the volume of air inhaled and exhaled during a particular period of time. Minute ventilation correlates well with the patient""s metabolic need for an increased heart rate over a range of heart rates. A minute ventilation signal can be obtained by measuring transthoracic (across the chest or thorax) impedance. Transthoracic impedance provides respiratory or ventilation information, including how fast and how deeply a patient is breathing.
A component of transthoracic impedance varies as the patient inhales and exhales. Ventilation (e.g., breathing rate, which is also referred to as xe2x80x9cventilation ratexe2x80x9d or xe2x80x9cVRxe2x80x9d, and breathing volume, which is also referred to as xe2x80x9ctidal volumexe2x80x9d or xe2x80x9cTVxe2x80x9d) information is included in the impedance signal. A minute ventilation signal (also referred to as xe2x80x9cminute volumexe2x80x9d or xe2x80x9cMVxe2x80x9d) signal is derived from the impedance signal, as illustrated by Equation 1. MV measures air flow rate (e.g., liters per minute), TV measures volume per breath (e.g., liters per breath), and VR measures breathing rate (e.g., breaths per minute).
MV=TVxc3x97VRxe2x80x83xe2x80x83(1)
A larger MV signal indicates a metabolic need for an increased heart rate, and the pacing rate can be adjusted accordingly by a cardiac rhythm management device. For example, one approach for measuring transthoracic impedance is described in Hauck et al., U.S. Pat. No. 5,318,597 entitled xe2x80x9cRATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE CONTROL ALGORITHM USING TRANS-THORACIC VENTILATION,xe2x80x9d assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. However, many problems must be overcome to provide the most effective cardiac rhythm management therapy to the patient in a device that can remain implanted in the patient for a long period of time before requiring a costly surgical explantation and replacement procedure.
First, ventilation information included in the transthoracic impedance signal is confounded with a variety of extraneous signals that makes the ventilation information difficult to detect. For example, as the heart contacts during each cardiac cycle, its blood volume changes, contributing to a significant change in the transthoracic impedance signal that is unrelated to the ventilation information. The change in the transthoracic impedance signal due to blood volume changes resulting from heart contractions is referred to as cardiac xe2x80x9cstroke volumexe2x80x9d or xe2x80x9cstrokexe2x80x9d signal. Moreover, the frequencies of the heart contractions (e.g., 1-3 Hz) are extremely close to the frequency of the patient""s breathing (e.g., under 1 Hz). This complicates separation of the stroke signal and the ventilation signal.
Furthermore, the frequency of the stroke and ventilation signals changes according to the patient""s activity. For example, a resting patient may have a heart rate of 60 beats per minute and a ventilation rate of 10 breaths per minute. When exercising, the same patient may have a heart rate of 120 beats per minute and a ventilation rate of 60 breaths per minute. The changing frequencies of the stroke and ventilation signals further complicates the separation of these signals.
Another aspect of heart contractions also masks the ventilation signal. Heart contractions are initiated by electrical depolariztions (e.g., a QRS complex) resulting from paced or intrinsic heart activity. Such electrical heart activity signals may be detected during the measurement of transthoracic impedance. This further diminishes the accuracy of the transthoracic impedance measurement, and increases the difficulty of obtaining accurate ventilation information.
A further problem with certain other minute ventilation based cardiac rhythm management devices results from the use of a relatively high amplitude current pulse (e.g., 1 milliampere) to detect transthoracic impedance. Using high amplitude stimuli wastes power, risks capturing the heart (i.e., evoking a contraction), may trigger false detection of intrinsic heart activity by the pacemaker""s sense amplifiers, and may produce a confusing or annoying artifact on electrodiogram (ECG) traces or other diagnostic equipment.
Thus, there is a need for a cardiac rhythm management device that effectively manages the patient""s heart rate based on an accurate inidication of metabolic need. Such a cardiac rhythm management device must be sufficiently robust to operate in the presence of extraneous noise signals that confound the indication of metabolic need. There is a further need for such a device to operate at low power consumption, in order to maximize the usable life of the battery-powered implantable device.
The present invention provides, among other things, a method of determining transthoracic impedance in a cardiac rhythm management device. A multiple phase stimulus is repeatedly delivered to a thorax region of a patient. More than one phase of each multiple phase stimuli is demodulated to obtain sample points of a response signal including transthoracic impedance information.
In one embodiment, a response to each phase is sampled, weighted to obtain a filtering function, and combined. In another embodiment, a rate of delivering cardiac rhythm management therapy is adjusted based on ventilation information included in the transthoracic impedance information of a plurality of the sample points. In another embodiment, a noise-response function inhibits rate-adjustment if the transthoracic impedance signal is too noisy. In a further embodiment, a interference avoidance function delays delivery of the multiple phase stimulus to avoid simultaneous occurrence with an interfering signal (e.g., a telemetry signal).
Another aspect of the invention includes a method of determining transthoracic impedance in a cardiac rhythm management device that includes delivering stimuli to a thorax of the patient, sensing a response signal including transthoracic impedance information, attenuating a component of the response signal having frequencies above a lowpass cutoff frequency, and adaptively basing the lowpass cutoff frequency on a heart rate, and independent of a breathing rate signal, from the patient.
In one such embodiment, a cardiac stroke signal is attenuated to obtain ventilation information. The lowpass cutoff frequency is adaptively selected to be below the heart rate by selecting between a number of discrete lowpass cutoff frequencies, each lowpass cutoff frequency corresponding to a particular range of values of the heart rate.
In a further embodiment, the method includes detecting peaks and valleys of the response signal. Differences between peaks and valleys of the response signal provide tidal volume data points, which are integrated for a predetermined period of time to obtain minute ventilation data points. A rate of delivering cardiac rhythm management therapy is adjusted based on the minute ventilation data points. Alternatively, breath-by-breath minute ventilation data points are obtained. Instead of performing the integration, time differences between the peaks and valleys of the response signal provide respiration period data points corresponding to the tidal volume data points. The tidal volume data points are divided by the corresponding respiration period data points to obtain minute ventilation data points, upon which a rate of delivering cardiac rhythm management therapy is adjusted.
Another aspect of the present invention includes a cardiac rhythm management device. The device includes an exciter, adapted to be coupled to a thorax of a patient for repeatedly delivering a multiphase stimulus thereto. A signal processor includes a receiver for obtaining trasthoracic impedance information responsive to the stimuli. A demodulator, included in the signal processor, includes sampling elements for demodulating the transthoracic impedance in response to different phases of the multiphase stimulus. A therapy circuit is adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto. A controller is coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the transthoracic impedance.
In one embodiment, the device includes a noise-reversion circuit that inhibits rate-adjustment if the transthoracic impedance signal is too noisy. In a further embodiment, the device is included within a cardiac rhythm management system that also includes an endocardial lead, carrying first and second electrodes, and a housing including third and fourth electrodes.
Another aspect of the invention includes a cardiac rhythm management device that includes an exciter for delivering stimuli to a thorax. A signal processor includes a receiver for obtaining a transthoracic impedance responsive to the stimuli. The signal processor extracts ventilation information from the transthoracic impedance. The signal processor includes an adaptive lowpass filter for removing a cardiac stroke component of the transthoracic impedance signal. A cutoff frequency of the adaptive lowpass filter is adaptively based on a heart rate signal of the patient. The cutoff frequency of the adaptive lowpass filter is independent of a breathing rate signal from the patient. A therapy circuit is adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto. A controller is coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the ventilation information.
The present invention provides, among other things, a cardiac rhythm management system, device, and methods that sense transthoracic impedance and adjust a delivery rate of the cardiac rhythm management therapy based on information extracted from the transthoracic impedance. The present invention effectively manages the patient""s heart rate based on an accurate indication of metabolic need. It provides robust operation in the presence of extraneous noise signals that confound the indication of metabolic need. It also provides low power consumption, increasing the usable life of the battery-powered implantable device. Other advantages will be apparent upon reading the following detailed description of the invention, together with the accompanying drawings which form a part thereof.